Carbohydrates constitute the most abundant organic compounds on earth. However, much of this carbohydrate is sequestered in complex polymers including starch (the principle storage carbohydrate in seeds and grain), and a collection of carbohydrates and lignin known as lignocellulose. The main carbohydrate components of lignocellulose are cellulose, hemicellulose, and pectins. These complex polymers are often referred to collectively as lignocellulose.
Bioconversion of renewable lignocellulosic biomass to a fermentable sugar that is subsequently fermented to produce alcohol (e.g., ethanol) as an alternative to liquid fuels has attracted an intensive attention of researchers since 1970s, when the oil crisis broke out because of decreasing the output of petroleum by OPEC. Ethanol has been widely used as a 10% blend to gasoline in the USA or as a neat fuel for vehicles in Brazil in the last two decades. More recently, the use of E85, an 85% ethanol blend has been implemented especially for clean city applications. The importance of fuel bioethanol will increase in parallel with increases in prices for oil and the gradual depletion of its sources. Additionally, fermentable sugars are being used to produce plastics, polymers and other biobased products and this industry is expected to grow substantially therefore increasing the demand for abundant low cost fermentable sugars which can be used as a feed stock in lieu of petroleum based feedstocks.
The sequestration of such large amounts of carbohydrates in plant biomass provides a plentiful source of potential energy in the form of sugars, both five carbon and six carbon sugars that could be utilized for numerous industrial and agricultural processes. However, the enormous energy potential of these carbohydrates is currently under-utilized because the sugars are locked in complex polymers, and hence are not readily accessible for fermentation. Methods that generate sugars from plant biomass would provide plentiful, economically-competitive feedstocks for fermentation into chemicals, plastics, such as for instance succinic acid and (bio) fuels, including ethanol, methanol, butanol synthetic liquid fuels and biogas.
Regardless of the type of cellulosic feedstock, the cost and hydrolytic efficiency of enzymes are major factors that restrict the commercialization of the biomass bioconversion processes. The production costs of microbially produced enzymes are tightly connected with a productivity of the enzyme-producing strain and the final activity yield in the fermentation broth.
In spite of the continued research of the last few decades to understand enzymatic lignocellulosic biomass degradation and cellulase production, it remains desirable to discover or to engineer new highly active cellulases and hemicellulases. It would also be highly desirable to construct highly efficient enzyme compositions capable of performing rapid and efficient biodegradation of lignocellulosic materials, in particular such cellulases and hemicellulases that have increased thermostability.
Such enzymes may be used to produce sugars for fermentation into chemicals, plastics, such as for instance succinic acid and (bio) fuels, including ethanol, methanol, butanol, synthetic liquid fuels and biogas, for ensiling, and also as enzyme in other industrial processes, for example in the food or feed, textile, pulp or paper or detergent industries and other industries.